The invention relates to a method and apparatus for acoustic examination of a medium to locate targets having acoustic impedance that is different from that of the surrounding medium. The term "acoustic" should be understood broadly, without being limited to frequencies that are audible.
The invention is applicable in numerous fields, and particular mention may be made of the following:
detecting and locating the defects such as faults, cracks, splits and non-uniformities of cristallographic structures, in a variety of materials such as metals, composites and ceramics, in objects of arbitrary shape, PA1 searching for solid objects in the sea or in sediments at the bottom of a body of water; PA1 locating non-uniformities in the ground; and PA1 locating objects to be grasped by a robot. PA1 (a) a highly divergent sound beam is injected into the medium from at least one transducer, PA1 (b) the signals reflected by the medium and received by a plurality of transducers in an array are picked up; PA1 (c) a time window or gate is used to select echoes coming from a particular zone of said medium, and the echoes are stored; PA1 (d) the signals are time-reversed and they are re-emitted; and PA1 (e) the signals newly reflected by said medium are stored and the time-reversal operation is repeated.
A method of non-destructive acoustic examination of a medium is already known (EP-A-0 383 650) enabling reflecting targets to be located therein, in which,
A small number of interations suffices to bring out only those signals that are reflected by the largest reflector in the medium or by the most highly reflective point of an extended reflecting target, the signals being in the form of a set having a marked maximum on each of the receive paths corresponding to respective transducers in the array.
In particular implementations of the method defined above, the time distribution of the signal maxima is stored for subsequent use. In another implementation, the spread of the signals in time is determined by cross-correlation between pairs of signals. Nevertheless, that requires a large amount of calculations. No immediately understandable indication is directly obtained concerning the location of the target (or targets) presence in the medium from either method.